By Irene Klotz
CAPE CANAVERAL, Florida (Reuters) - Four U.S. astronauts in bright orange spacesuits climbed aboard their spaceship on Thursday to practice the launch of NASA's last space shuttle on a cargo run to the International Space Station.
The flight of Atlantis is targeted for liftoff on July 8 and will close out NASA's 30-year-old space shuttle program. The program is ending due to high operating costs and to free up funds to develop spacecraft that can travel beyond the space station, a $100 billion project of 16 nations that orbits 220 miles above Earth.
The shuttle will deliver a year's worth of food, clothing, science gear and equipment to the station in case commercial cargo carriers hired to replace the shuttle are delayed.
NASA limited the number of astronauts on the final shuttle flight to four, rather than the six or seven typically assigned. That will accommodate the smaller Russian Soyuz capsules that will serve as the Atlantis crew's lifeboats.
Since the 2003 Columbia accident, NASA has had a second shuttle in waiting, ready to mount a rescue mission should a crew find their ship too damaged by launch or orbital debris strikes to fly back through the atmosphere for landing.
Columbia was destroyed as it glided through the atmosphere for landing. Its heat shield was damaged by a piece of foam insulation that fell off the fuel tank and hit the shuttle during launch. Seven astronauts were killed when the shuttle broke apart over Texas and Louisiana.
There are no more shuttles available for rescue missions, so the last shuttle crew would rely instead on the three-seater Soyuz capsules to ferry them home, one at a time over a year.
Being short-handed means the crew had to revamp how they operate the shuttle and handle cargo transfers to the space station.
"The overall workload is pretty high," said Atlantis astronaut Sandra Magnus, who will oversee the crew's work on the station.
"We've had to do a lot more cross-training than normal for a shuttle crew," she added. "You end up being a little bit more of a jack-of-all-trades."
Speaking to reporters by the launch pad where Atlantis is being prepared for flight, shuttle commander Chris Ferguson said on Wednesday that he and his crewmates have had little time to dwell on the significance of the final shuttle mission.
"I hate to see the space shuttle go away, I really do, but I think that there are some events historically that have led to this and it's probably appropriate," Ferguson said.
NASA will rely on Russian crew transports while it supports private U.S. industrial efforts to develop space taxis, but it will be at least five years before those are ready to fly.
Cargo hauls are being turned over to Space Exploration Technologies and Orbital Sciences Corp, which are expected to start deliveries to the space station next year. Station freight services also are provided by the Russian, Japanese and European space agencies.
NASA will set a firm launch date for Atlantis on June 28. The flight is scheduled to last 12 days. If the shuttle flies as planned on July 8, landing day would fall on July 20, the anniversary of the Apollo 11 moon landing.
(Editing by Jane Sutton and Eric Walsh)